The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an enhanced cutting structure for such bits. Still more particularly, the invention relates to the placement of cutter elements on the rolling cone cutters at locations that increase bit durability and rate of penetration and enhance the bit""s ability to maintain gage.
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or xe2x80x9cgagexe2x80x9d of the drill bit.
A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones. Such bits typically include a bit body with a plurality of journal segment legs. The cutters are mounted on bearing pin shafts which extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit. The drilling fluid carries the chips and cuttings in a slurry as it flows up and out of the borehole. The earth disintegrating action of the rolling cone cutters is enhanced by providing the cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as xe2x80x9cTCIxe2x80x9d bits, while those having teeth formed from the cone material are known as xe2x80x9csteel tooth bits.xe2x80x9d In each case, the cutter elements on the rotating cutters functionally breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a xe2x80x9ctripxe2x80x9d of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (xe2x80x9cROPxe2x80x9d), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements (both steel teeth and TCI inserts) upon the cutters greatly impact bit durability and ROP and thus are critical to the success of a particular bit design.
Bit durability is, in part, measured by a bit""s ability to xe2x80x9chold gage,xe2x80x9d meaning its ability to maintain a full gage borehole diameter over the entire length of the borehole. Gage holding ability is particularly vital in directional drilling applications which have become increasingly important. If gage is not maintained at a relatively constant dimension, it becomes more difficult, and thus more costly, to insert drilling apparatus into the borehole than if the borehole had a constant diameter. For example, when a new, unworn bit is inserted into an undergage borehole, the new bit will be required to ream the undergage hole as it progresses toward the bottom of the borehole. Thus, by the time it reaches the bottom, the bit may have experienced a substantial amount of wear that it would not have experienced had the prior bit been able to maintain full gage. This unnecessary wear will shorten the bit life of the newly-inserted bit, thus prematurely requiring the time consuming and expensive process of removing the drill string, replacing the worn bit, and reinstalling another new bit downhole.
To assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a heel row of hard metal inserts on the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to generally align with and ream the sidewall of the borehole as the bit rotates. The inserts in the heel surface contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, decreased ROP, increased loading on the other cutter elements on the bit, and may accelerate wear of the cutter bearing and ultimately lead to bit failure.
In addition to the heel row inserts, conventional bits typically include a gage row of cutter elements mounted adjacent to the heel surface but orientated and sized in such a manner so as to cut the corner of the borehole. In this orientation, the gage cutter elements generally are required to cut both the borehole bottom and sidewall. The lower surface of the gage row insert engages the borehole bottom while the radially outermost surface scrapes the sidewall of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located on the cones in rows disposed radially inward from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutter elements.
Differing forces are applied to the cutter elements by the sidewall than the borehole bottom. Thus, requiring gage cutter elements to cut both portions of the borehole compromises the cutter design. In general, the cutting action operating on the borehole bottom is typically a crushing or gouging action, while the cutting action operating on the sidewall is a scraping or reaming action. Ideally, a crushing or gouging action requires a tough insert, one able to withstand high impacts and compressive loading, while the scraping or reaming action calls for a very hard and wear resistant insert. One grade of tungsten carbide cannot optimally perform both of these cutting functions as it cannot be as hard as desired for cutting the sidewall and, at the same time, as tough as desired for cutting the borehole bottom. As a result, compromises have been made in conventional bits such that the gage row cutter elements are not as tough as the inner row of cutter elements because they must, at the same time, be harder, more wear resistant and less aggressively shaped so as to accommodate the scraping action on the sidewall of the borehole.
Accordingly, there remains a need in the art for a drill bit and cutting structure that is more durable than those conventionally known and that will yield greater ROP""s and an increase in footage drilled while maintaining a full gage borehole. Preferably, the bit and cutting structure would not require the compromises in cutter element toughness, wear resistance and hardness which have plagued conventional bits and thereby limited durability and ROP.
The present invention provides an earth boring bit for drilling a borehole of a predetermined gage, the bit providing increased durability, ROP and footage drilled (at full gage) as compared with similar bits of conventional technology. The bit includes a bit body and one or more rolling cone cutters rotatably mounted on the bit body. The rolling cone cutter includes a generally conical surface, an adjacent heel surface, and preferably a circumferential shoulder therebetween. A row of gage cutter elements are secured to the cone cutter and have cutting surfaces that cut to full gage. The bit further includes a first inner row of off-gage cutter elements that are secured to the cone cutter on the conical surface and positioned so that their cutting surfaces are close to gage, but are off-gage by a distance D that is strategically selected such that the gage and off-gage cutter elements cooperatively cut the corner of the borehole.
According to the invention, the cutter elements may be hard metal inserts having cutting portions attached to generally cylindrical base portions which are mounted in the cone cutter, or may comprise steel teeth that are milled, cast, or otherwise integrally formed from the cone material. The off-gage distance D may be the same for all the cone cutters on the bit, or may vary between the various cone cutters in order to achieve a desired balance of durability and wear characteristics for the cone cutters. The gage row cutter elements may be mounted along or near the circumferential shoulder, either on the heel surface or on the adjacent conical surface.
The number of gage row cutter elements may exceed the number of first inner row cutter elements. In such embodiments, the gage row inserts will be positioned such that two or more of the gage cutter elements are disposed between a pair of first inner row cutter elements.
Where the gage cutter elements and first inner row off-gage cutter elements are inserts, the ratio of the diameter of the gage row inserts to the diameter of the off-gage inserts is not greater than 0.75 for certain preferred embodiments of the invention.
In another embodiment, the cutting profiles of the gage and off-gage cutter elements will overlap when viewed in rotated profile such that the ratio of the distance of overlap to the diameter of the gage row inserts is greater than 0.4.
In other embodiments of the invention, the extension of the gage cutter elements and off-gage cutter elements will define a step distance, where the ratio of the step distance to the extension of the gage cutter elements will be greater than 1.0 for TCI bits having an IADC formation classification within the range of 41 to 62. The invention may also comprise steel tooth bits where the ratio of step distance to the extension of the gage cutter elements is greater than 1.0.
The invention permits dividing the borehole corner cutting load among the gage row cutter elements and the first inner row of off-gage cutter elements such that the first inner row of cutter elements primarily cuts the bottom of the borehole, while the gage cutter elements primarily cut the borehole sidewall. This positioning enables the cutter elements to be optimized in terms of materials, shape, and orientation so as to enhance ROP, bit durability and footage drilled at full gage.
In still another alternative embodiment of the invention, the bit includes a heel row of cutter elements having cutting surfaces that cut to full gage, and a pair of closely-spaced rows of off-gage cutter elements. The off-gage cutter elements in the first of the closely spaced rows have cutting surfaces that are off-gage a first predetermined distance. The cutter elements in the second row of the pair have cutting surfaces that are off-gage a second pre-determined distance, the first and second distances being selected such that the first and second rows of off-gage cutter elements cooperatively cut the borehole corner. This embodiment also provides a pair of closely spaced rows of cutter elements that are positioned to share the borehole corner cutting duty. This permits the elements to be optimized for their particular duty, leading to enhancements in ROP, bit durability and ability to hold gage.